Nasty Looking Decay on Friend's Fish

My friend has a bristlenose that's had an issue for a bit now. A couple weeks back he showed me it has some mild white marks on it's scales and I said it's hopefully just surface damage but keep an eye on it. He's just updated me on it, and it's gotten a lot worse. I'm trying to identify the issue and if there's a good action for him to take but I'm struggling. Picture is the most recent one he showed me

Ouch, that looks pretty nasty.


Worth a look
I would definitely suggest wiping down the glass, doing a large water change, and immediately isolating the fish if they can (Just a summarised version of Colin’s basic fish first aid ). Also, get them to test the water and send us the results. I’m definitely not a professional, so hopefully you can get some expert answers. Do any of the other fish have it?

Good luck to you and your friend! (And welcome to the forum!)

Thanks for the response. I already asked him to do a test and some cleaning, he'll be doing that when he gets a chance. As far as I'm aware that's the only fish affected. I'll update with test results once I have them.

bacterial infection.
clean tank, glass, gravel and filter.
treat with a medication that kills fungus and bacteria.